Alex Smith injury: San Francisco 49ers QB leaves vs. Rams after big hit

Alex Smith took a big hit in the second quarter from St. Louis Rams linebacker Jo-lonn Dunbar, and while he tried to stay in the game the San Francisco 49ers quarterback went out shortly thereafter, replaced by young backup Colin Kaepernick. After the 49ers' drive Smith was declared out for the game with what sideline reporters confirmed was a concussion.

Here's how it happened—Smith escaped the pocket and slid awkwardly to the ground after making a short gain, but with his back to Dunbar and his lower body already on the ground he was folded up by the contact. He stayed in the game and even got the 49ers on the board with their first touchdown of the afternoon, to Michael Crabtree, but he left afterward to be evaluated by San Francisco's medical personnel in the locker room.

The Rams lead the 49ers 14-7 through most of the second quarter. Kaepernick comes into the game with 13 rushes for 111 yards and five passes—for 89—in nine attempts as a highly effective change-up of a quarterback. Not he's got a much bigger responsibility on his hands.